Don't feel constrained against introducing new content in that essay. Remember, you only get so many words to convey all of who you are, your hopes, dreams, etc. Spending an entire essay covering material the adcom can learn elsewhere in the app is not making full use of the space.
Of course, expanding on activities listed in the data form is a great approach, but if there's a great story that does a lot to explain who you are, don't leave it out because it's not told elsewhere in the app.